Ultrafine metal particles of the order of nanometers exhibit properties different from those of general materials, such as large specific surface areas and specific properties depending on their quantum size effect. Therefore, their study and use have been forwarded in a variety of fields such as of electronic materials, magnetic materials, optical materials, osmotic films, catalytic materials, antibacterial agents and the like.
The ultrafine metal particles, on the other hand, are highly active, unstable, cannot maintain the state of ultrafine particles their form of a simple substance and, therefore, tend to aggregate together. In the use where optical properties are required, therefore, the ultrafine metal particles are accompanied by such problems as low transparency, poor properties or without at all exhibiting properties.
In the step of preparing ultrafine metal particles, further, the handling property decreases causing, therefore, a decrease in the productivity. Thus the ultrafine metal particles had not still been fully satisfactory because of their poor transparency and poor productivity. To solve these problems, therefore, some proposals have been made as described below.
For instance, a patent document 1 listed below is proposing ultrafine metal composite particles forming a shell of an organometal compound surrounding the metal. The present applicant has earlier proposed a resin composition containing ultrafine metal particles formed by blending a resin with a fatty acid metal salt followed by heating (patent document 2). There has, further, been proposed a method of producing ultrafine metal particles by mixing a metal salt and a chemically adsorptive organic compound containing a functional group together, and heating and reacting them together (patent document 3).
There are many prior arts related to the use of optical materials, such as acrylic resin containing a quaternary ammonium compound as an antibacterial agent (patent document 4), a photocurable composition for antibacterial coating comprising a photocurable acrylic resin that contains a silver salt (patent document 5), and a resinous formed body such as boards having antibacterial power for protecting various display devices, comprising a photocurable resin and an antibacterial agent and/or an anti-molding agent contained therein (patent document 6).